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Article Series 1: The Blood and the Immune System

The Components of the Blood and the Immune System

We all have a general idea of what blood is. The heart pumps it
through our body, and we need it to live. It is used to swear
eternal friendship. We use it to save lives. If we are sick, the
doctor can test our blood to find out what is wrong. But how can
the blood reflect how we are sick, and why is it so essential to
life? The biology of the blood is fascinating, and in the
following educational article series, I will introduce what
blood really consists of, and discuss the function, the
development, and some of the diseases of our blood cells and our
immune system.

What
are the components of blood?

Your blood makes up about 7% of your
body weight, and in an adult amounts to around 5 liters (= 1.3
gallons)1. If you lose too much blood, for instance
during a trauma or an accident, it can be life-threatening,
and you may need a blood transfusion. We are able to donate up
to half a liter, which is about 10% of our total blood volume,
however such blood donation requires overall good health, and
time to rest and recover afterwards. Roughly half of the blood
volume consists of various blood cells, while the other half
is blood plasma, the liquid which enables your blood to flow
throughout your body. Every component in the blood has their
own critical function, which will be introduced in this
article.

The
blood plasma consists of approximately 90 % water1

Water is critical for life. A human can
survive for days and even weeks without food, but only a few
days without water. The reason is that water is a resource
that constantly recycles. We lose water from our body both
through urine and as evaporation from our skin through sweat.
Neither of these processes are something we can consciously
control, but they are important processes for temperature
control as well as getting rid of waste products. On the other
hand, our water intake is under our control. We get water from
what we drink, but also through food. Mild dehydration may
lead to headache, overheating, or dizziness, but is not
life-threatening under normal circumstances. In cases of
extreme dehydration, you can get liquidthrough intravenous transfusions directly into your
blood.

The
blood plasma contains various soluble components

In addition to giving blood its
fluidity, so that blood cells can be transported throughout
the body, water is also important as a solvent for transport
of nutrients and waste products. Minerals, vitamins, glucose,
and various types of proteins, along with the water, make up
the blood plasma. Although the color of the blood is red, the
color of the blood plasma is actually yellow. The red color
comes from the large amount of red bloods cells, as will be
described below. The yellow color of the blood plasma comes
from the various water-soluble components, such as nutrients
and various signaling molecules. Furthermore, your blood is
the carrier of various waste products, that are filtered out
from the blood to the urine through the kidneys. In addition,
the blood contains various proteins that have both structural
as well as regulating or signaling roles. One type of
important structural proteins are the coagulation factors that
are required for proper blood clotting. Insulin is an example
of signaling molecule. People suffering from diabetes must
closely monitor and adjust the glucose and insulin levels in
their blood, to ensure a proper balance.

The
cells in our blood

The cells in our blood are divided into
two main types: The red blood cells, and the white blood
cells. In addition, there are specialized cell fragments,
called platelets, that are derived from a specific type of
white blood cells, the megakaryocytes. The red blood cells
(RBC, also called erythrocytes) take up about 45% of the total
blood volume1. The red color is due to abundant
amounts of the protein hemoglobin, which binds and transports
oxygen from the lungs throughout our body. The white blood
cells are critical for our immune system, which can broadly be
divided into the innate and the adaptive immune
system. The innateimmune system
recognizes patterns that are associated with pathogens,
and mounts a fast reaction towards infections. The adaptive
immune cells recognize specific eptiopes,
and can be educated to recognize epitopes associated with
disease. The response of adaptive immune cells are initially
slower, but the education leads to a "memory" so that upon
later encounters, we can quickly recognize and eliminate the
threat. Immunization is based on the ability of the adaptive
immune system to recognize the pathogen and develop a
protective "memory" or immunization. Lastly, the platelets,
also called thrombocytes, are not cells, but rather cell
fragments. They are critical for blood coagulation, to ensure
that upon a cut or damage to a blood vessel, the bleeding will
stop.

Summary

Human blood has many well defined
components, and is the major transport system of both
nutrients, signaling molecules, waste products and immune
cells. Therefore, a simple blood sample can reveal many types
of disease or unbalance in the body, and thereby help with
initial diagnosis.

Glossary:

Adaptive Immune System: Is the
part of the immune system that can adapt to and "remember" the
pathogen based on "memory". This feature is unique to higher
vertebrates, and is based on a clonal expansion of cells that
are able to react to a pathogen. Immunizations take advantage
of the adaptive immune system, by introducing the body to
potential pathogens (for instance the inactivated flu virus)
to elicit a memory response to protect you from future
exposure.

Epitope: A surface that can be
recognized by the immune system. It is also called an
antigenic determinant, as it is a part of an antigen, the unit
that an antibody or immune receptor recognize. The epitope can
be a linear sequence of for instance amino acids (a small part
of a protein, called a peptide), or it can be a 3-dimensional
surface composed of different parts.

Innate Immune System: Is the part
of the immune system that is innate, that we are born with. It
generally recognizes so-called Pathogen-Associated Molecular
Patterns (PAMPs), for instance common repetitive structures on
the surface of bacteria and viruses.

Pathogen: A foreign organism that
leads to damage or disease (pathology). Bacteria, virus,
parasites are all examples of potential pathogens. However,
the commensal bacteria in your gut, or the cultures in
yoghurt, are not pathogens.

Acknowledgement: Thanks to
writer/director Jennifer
Sharp for critical review of and valuable feedback for
this article.

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Latest Updates:

10/13-2012: Educational article: The Components of the
Blood and the immune System.